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The Effect of Plating, Surface Finish, and Bond Line Thickness on AuSn Solder Joints

Keywords: AuSn, molybdenum-copper, copper-tungsten

AuSn solder provides a conductive but rigid joint for semiconductor die attachment to molybdenum-copper or tungsten-copper heat sinks. The thermal and electrical resistance and strength of the bond are critical to high power, high reliability electronics and diode lasers. There exists a range of design approaches to using AuSn solder, and additional guidelines would be valuable for improving performance.
In order to understand the constraints on AuSn bonds, an experimental matrix to measure the effect of some parameters on the joint strength and electrical resistance of AuSn bond joints was performed. Molybdenum-copper test slugs were manufactured with a range of surface finishes. Nickel followed by Au was plated over the test parts at various thicknesses. After plating, AuSn was applied either in a thin, vacuum deposited layer or as a thicker preform on one side. Two parts were soldered together and the bond strength and electrical resistance of each joint was measured.